the landlord failed to return 2700 zl
Normal enough in PL. You may as well cut your losses and forget it. If you're still angry, write to the tax office telling them, how much you paid your landlord, how long you were there, and (if you've got such a thing) proof of payments. Include a copy of the contract if you still have it.
Unfortunately Poland doesn't have a system like the UK where deposits are held centrally and any landlord has to prove their claim if they want to keep all or part. And if such a system existed here, landlords would do whatever they could to illegally get round it.
Given their reluctance to part with money, it's best to time your departure discreetly so the deposit acts as the last payment. They hate that but wouldn't think twice about doing it themselves.